This invention relates to a hydraulic tensioner for keeping constant tension in a belt or a chain for driving camshafts.
An ordinary hydraulic tensioner includes a hydraulic damper which bears fluctuating loads applied from a belt or a chain while absorbing vibration of the belt or chain. The hydraulic damper includes a check valve having a check ball which can be moved into and out of contact with a valve seat. When a pushrod or a plunger of the damper is pushed in, the check ball is brought into close contact with the valve seat, thereby sealing a pressure chamber of the damper, which is filled with hydraulic oil, to substantially prevent the hydraulic oil in the pressure chamber from flowing out (as disclosed in FIGS. 12 and 13 of JP patent publication 10-325448).
In such a hydraulic tensioner, from the belt or chain, fluctuating loads are repeatedly applied to the hydraulic damper, thus repeatedly moving the check ball between its closed and open positions, i.e. into and out of contact with the valve seat. Since the check ball and the valve seat repeatedly contact each other, both of them are subjected to treatment for improving wear resistance. Specifically, the check ball is usually formed of a bearing steel and is hardened. The valve seat is usually formed of a forgeable alloyed steel for a mechanical structure such as chrome steel, chrome molybdenum steel or other steel for carburizing, and is carburized and tempered.
If the check ball and the valve seat have the same hardness, the valve seat usually becomes worn locally faster than the check ball because the valve seat is immovable and thus brought into contact with the ball at the same point, while the check ball is brought into contact with the valve seat at different points because the ball can turn. If the valve seat is worn locally, it becomes difficult to stop flow of hydraulic oil because the ball is not in close contact with the seat. Or the check ball may be trapped in the seat and become inseparable from the seat. In either case, life of the tensioner shortens.
To solve this problem, the applicant of this invention has proposed a hydraulic tensioner having a check valve of which the valve seat has a higher hardness than the check ball (JP patent application 2001-166659). In this arrangement, however, depending upon heat treatment conditions of the valve seat, carbides tend to deposit, causing progression of wear. Thus, no significant extension of life of the tensioner, compared with existing ones, was necessarily achieved.
An object of the invention is to improve wear resistance of a valve seat of a check valve in a hydraulic autotensioner, thereby prolonging life of this tensioner.